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Effects of Eu modification and heat treatment on microstructure and mechanical properties of hypereutectic Al–Mg2Si composites

To modify the coarse primary Mg2Si phase and improve mechanical properties of Mg2Si reinforced aluminum matrix composites, hypereutectic Al–18Mg2Si composites with different Eu contents were fabricated by permanent mold casting and extrusion followed by heat treatment. Results show that the as-cast Al–18Mg2Si composites consisted of primary Mg2Si and eutectic structure (Al + Mg2Si). Eu addition can obviously modify the primary Mg2Si morphology from dendrite to flat truncated octahedron shape, and the average size also decreased from ∼90 μm to ∼16 μm with increasing Eu content from 0 to 0.1%. But further increasing Eu content would cause over-modification. After T6 treatment, sharp corners of primary Mg2Si were spheroidized and the average size of primary Mg2Si were further decreased. DTA result shows the nucleation temperature of primary Mg2Si increased by Eu addition. Al2Si2Eu phase can act as the heterogeneous nucleation substrate for the primary Mg2Si phase and thus refine the size of primary Mg2Si particle. TEM results show the segregation and/or selective adsorption of Eu on the {100} facets of primary Mg2Si suppress the growth of primary Mg2Si, and thus refine and modify the composites, which proposed to the dominant reason for the refinement and modification of the primary Mg2Si phase. Tensile tests show that the mechanical properties enhanced with increasing Eu content. The elongation and ultimate tensile strength of the T6-treated Al–18Mg2Si-0.1Eu composite improved to 5.2% and 241 MPa, which enhanced 225% and 91% compared to the unmodified as-cast Al–18%Mg2Si composite. It is concluded that the morphology transition of primary Mg2Si from dendrite to tetrakaidecahedron and the refinement of primary Mg2Si size with Eu modification, the passivation of the sharp corners in primary Mg2Si particles as well as the appearance of β" precipitates all contribute to the improvement of the mechanical properties of the T6-treated Al–18Mg2Si composites.

The Role of Selenium on the Formation of Spheroidal Graphite in Cast Iron

Sulfur, an element that belongs to group 16 (chalcogens) of the periodic table, is an excellent promoter of nucleation substrates for graphite in cast iron. In ductile iron, sulfur favors a higher nodule count, which inhibits the risk of carbides and of microporosity. It is reasonable to expect that other elements from group 16, such as selenium or tellurium, play similar roles in the nucleation of graphite. The objective of this paper was to investigate the effect of selenium on the process of graphite formation. Thermal analysis cups were poured to evaluate the nodule count and size distribution. Some of the cups were not inoculated, while others were inoculated with a Ce-bearing inoculant, or with the Ce inoculant and additions of Se. Cross-shaped castings were also poured to quantify the microporosity regions by tomography. It appears that selenium additions modify the number and size of graphite particles, as well as the volume of microshrinkage. Direct correlations between these three parameters were found. Advanced Extensive Field Emission Gun Scanning Electron Microscope (FEG-SEM) techniques were used to identify the nature of the main nucleation compounds. Selenides, combined with Mg and rare earths, were observed to serve as nuclei for graphite. Their presence was justified by thermodynamics calculations.

Refining and modification effects of (Al, Zr, Si)–Al4Sr on Al–7Si–0.5 Mg alloy

The Al–9Zr-0.9Sr master alloy was used to refine and modify the Al–7Si-0.5 Mg alloy, and to improve the refining and modification effects, the Al–9Zr-0.9Sr master alloy was carried out melt spinning treatment. The second phase particles in the ribbon are smaller and more evenly distributed than those in the ingot. The Al–Zr–Sr inoculation showed excellent refining and modification effects on Al–7Si-0.5 Mg alloy, and in the range of 1 wt.%∼4 wt.%, the refining effect was gradually enhanced with the increase of the inoculant dosage. The morphology and size of particles in inoculant have significant effects on the refinement and modification effects of the inoculant. The X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and selected area electron diffraction (SAED) results showed that in Al–7Si-0.5 Mg matrix alloy, (Al, Zr, Si) instead of Al3Zr was used as the heterogeneous nucleation substrate of α-Al to refine the grains. The Sr element in the master alloy was mainly used to modify eutectic silicon, and melt spinning could effectively improve its modification effect on eutectic silicon. With the increase of inoculant dosage, the mechanical properties of the alloy were gradually improved. However, when the dosage was increased to 4 wt.%, some large rod-like particles appear due to the excessive Zr element, resulting in a decrease in elongation.

Effect of CeO2 Content on Microstructure and Properties of SiCp/Al-Si Composites Prepared by Powder Metallurgy.

The effect of CeO2 content on the microstructure and properties of SiCp/Al-Si composites prepared by powder metallurgy was studied, and the mechanism of CeO2 in composites was deeply analyzed. The results show that the addition of the appropriate amount of CeO2 can refine the Si particles and improve the tensile properties of the SiCp/Al-Si composites. As the CeO2 content increases from 0 to 0.4 vol%, the particle size of the Si phase shows a tendency to decrease first and then increase, while the tensile strength, yield strength, and elongation of the composites show a trend of first increasing and then decreasing. When the CeO2 content is 0.2 vol%, the refining effect of CeO2 and the tensile properties of composites are the best. The fracture mode of SiCp/Al-Si composites with a rare earth addition is a mixed fracture. There are three main mechanisms for CeO2 in SiCp/Al-Si composites. One is when CeO2 serves as the nucleation substrate of Si phase to refine Si particles. The second is when CeO2 reacts with the alloying elements in the aluminum matrix to form a new phase, CeCu2Si2, which can not only play a role of dispersion strengthening, but also improve the bonding strength between Al matrix and Si particles. The third is the pinning effect of CeO2 and CeCu2Si2 particles on grain boundaries or phase boundaries to refine aluminum grains.

Preparation and characterization of Cf/Pollucite composites through geopolymer precursors

Continuous carbon-fibre-reinforced Cs-geopolymer composite (Cf/CsGP) were prepared, and its in-situ conversion was investigated during high-temperature treatments. The effect of treatment temperature on the thermal evolution process and mechanical properties of the resulting products were systematically evaluated. The results indicated that the crystallization temperature of Cf/CsGP composite was considerably delayed because the amorphous structure of carbon fibres was not conducive as a nucleation substrate for pollucite derived from the CsGP matrix. Moreover, the integrity of the corresponding resulting products derived from the Cf/CsGP composite were damaged due to thermal shrinkage that occurred during the high-temperature treatment process. When treatment temperature was ≤1200oC, the mechanical properties of the corresponding products exhibited an upward trend, which was ascribed to the improvement of the densification degree of the resulting composite and well interface-bonding state between carbon fibres and pollucite. However, the mechanical properties of the resulting composites decreased with the treatment temperature continued increased from 1200 to 1400oC. This phenomenon was attributed to the impairment of fibre properties caused by interfacial reactions.

Effect of Ti and Sb Elements Addition on the Microstructure and Corrosion Resistance of Hot-Dip Galvanized Zn-11Al-3Mg Alloy

The effect of Ti and Sb elements addition on the microstructures and corrosion resistance of hot-dip galvanized Zn-11Al-3Mg alloy was investigated by scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS), X-ray diffraction (XRD) and Electrochemical workstation, respectively. Results showed that the grain size of the alloy was obviously refined with the addition of Ti and Sb elements, due to Al3Ti phase as the nucleation substrate resulted in the transformation of Al-rich phase from dendrite to petal-like. In addition, the pitting corrosion resistance of the alloy has been significantly enhanced.

The influence of microbial mats on travertine precipitation in active hydrothermal systems (Central Italy)

AbstractThe study of hydrothermal travertines contributes to the understanding of the interaction between physico‐chemical processes and microbial mats in carbonate precipitation. Three active travertine sites were investigated in Central Italy to characterise the types of carbonate precipitates and the associated microbial mats at varying physico‐chemical parameters. Carbonate precipitated fabrics at the decimetre to millimetre‐scale and microbial mat composition vary with decreasing water temperature: (a) at relatively higher temperature (55–44°C) calcite and aragonite crystals precipitate on microbial mats of Chloroflexi and sulphur‐oxidizing microbes forming filamentous streamer fabrics with sparse cyanobacteria, (b) at intermediate temperature (44–40°C), rafts, coated gas bubbles and dendrites are associated with Spirulina cyanobacteria and other filamentous and rod‐shaped cyanobacteria, (c) low temperature (34–33°C) laminated crusts and oncoids forming in a terraced slope system are associated with diverse Oscillatoriales and Nostocales filamentous cyanobacteria, Spirulina and diatoms. At the microscale, carbonate precipitates are similar in the three sites consisting of prismatic calcite crystals organised in radial rosettes or fibrous aragonite spherulites (40–300 µm in diameter), overlying or embedded in Extracellular Polymeric Substances. Clotted peloidal micrite dominates at temperatures <40°C, also encrusting filamentous microbes. Carbonates are associated with gypsum crystals; extracellular polymeric substances are enriched in silicon, aluminium, magnesium, calcium, phosphorous and sulphur; authigenic aluminium‐silicates form aggregates on Extracellular Polymeric Substances. This study confirms that microbial communities in hydrothermal settings vary as a function of water temperature. Carbonate precipitates at the microscale are similar in the three settings, despite different microbial communities, suggesting that travertine precipitation, driven by carbon dioxide degassing, is influenced by biofilm extracellular polymeric substances acting as a substrate for crystal nucleation (Extracellular Polymeric Substances‐mediated mineralization) and affecting the resultant fabric types, independently from specific microbial community composition and metabolism.

Solid solution strengthening mechanism in high pressure die casting Al–Ce–Mg alloys

A series of Al-xCe (x = 2, 4, 6, 8, 10 wt%) and Al–8Ce-yMg (y = 0, 0.10, 0.25, 0.50, 0.75 wt%) alloys were prepared by high-pressure die casting. The introduced cerium element promoted the nucleation of α-Al grains. Al11Ce3 phase served as the heterogeneous nucleation substrate of α-Al due to the small lattice mismatch of 6.72%. According to the results calculated by Nelson-Riley extrapolation function, the lattice constant of α-Al increased from 4.0511 Å to 4.0540 Å with the increasing of Mg content from 0 wt% to 0.75 wt%. Due to the solid solution strengthening effect of Mg atoms, the yield strength of Al–8Ce-yMg alloys and the hardness of α-Al matrix in the Al–8Ce-yMg alloys showed a parabolically increasing tendency, from 92 MPa to 115 MPa and 0.502 GPa–0.575 GPa, respectively. The work hardening capacity of Al–8Ce-yMg alloys was improved by the solid solution of Mg with the work hardening exponent increasing from 0.21 to 0.27. The solid solution of Mg atoms reduced the stacking fault energy of Al–8Ce-yMg alloys and suppressed the dynamic recovery process of the alloys during deformation process, which promoted the formation of dislocation tangles and dislocation networks in the α-Al matrix.

Expanded MoSe2 Nanosheets Vertically Bonded on Reduced Graphene Oxide for Sodium and Potassium-Ion Storage.

The cost-efficient and plentiful Na and K resources motivate the research on ideal electrodes for sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs). Here, MoSe2 nanosheets perpendicularly anchored on reduced graphene oxide (rGO) are studied as an electrode for SIBs and PIBs. Not only does the graphene network serves as a nucleation substrate for suppressing the agglomeration of MoSe2 nanosheets to eliminate the electrode fracture but also facilitates the electrochemical kinetics process and provides a buffer zone to tolerate the large strain. An expanded interplanar spacing of 7.9 Å is conducive to fast alkaline ion diffusion, and the formed chemical bondings (C-Mo and C-O-Mo) promote the structure integrity and the charge transfer kinetics. Consequently, MoSe2@5%rGO exhibits a reversible specific capacity of 458.3 mAh·g-1 at 100 mA·g-1, great cyclability with a retention of 383.6 mAh·g-1 over 50 cycles, and excellent rate capability (251.3 mAh·g-1 at 5 A·g-1) for SIBs. For PIBs, a high first specific capacity of 365.5 mAh·g-1 at 100 mA·g-1 with a low capacity fading of 51.5 mAh·g-1 upon 50 cycles and satisfactory rate property are acquired for MoSe2@10%rGO composite. Ex situ measurements validate that the discharge products are Na2Se for SIBs and K5Se3 for PIBs, and robust chemical bonds boost the structure stability for Na- and K-ion storage. The full batteries are successfully fabricated to verify the practical feasibility of MoSe2@5%rGO composite.

Organic fouling control in reverse osmosis (RO) by effective membrane cleaning using saturated CO2 solution

Although reverse osmosis (RO) currently dominates the global desalination market, membrane fouling remains a major operational obstacle, which penalizes sustainable plant operation. This study explores a new membrane cleaning technique that uses a saturated CO2 solution to alleviate membrane fouling caused by organic matter, without any additional chemicals. When the CO2 saturated solution is injected into the membrane module at a given pressure, CO2 bubbles start nucleating throughout the membrane surface. This phenomenon is intensified underneath the deposited foulants. The porous structure of the foulants presents cavities, which are considered as imperfection sites that act as a substrate for CO2 bubbles nucleation, leading to an effective membrane cleaning. In this study, sodium alginate, a model polysaccharide, was mixed with different concentrations of Ca2+ to evaluate the cleaning efficiency of the CO2 technique under severe operating conditions when formed Ca2+/alginate fouling layers significantly impend the RO process performance. Furthermore, the effect of hydrodymamic conditions and CO2 saturation pressure on efficiency of permeate flux recovery and membrane morphology is also evaluated and the results are compared to those achieved with Milli-Q water and acidic solution at pH 4 cleanings. Better permeate flux recoveries were observed at higher Ca2+ concentrations comparing to fouling expriments at lower concentrations. The observed effect was attributed to a transition from the gel layer to a looser cake layer which makes CO2 bubble nucleation and subsequent permeate flux recovery more effective due to the presence of a larger number of CO2 nucleation sites as a result of a formation of more porous fouling structures. Permeate flux recovery increased with the increase in cleaning time, cross-flow velocity and CO2 saturation pressure.

A Critical Conception of Hot-Tearing Susceptibility: How Controlled Diffusion Solidification Enables Shape-Casting with Wrought Aluminum Alloys

Instrumented Constrained Rod Casting (CRC) method and the Controlled Diffusion Solidification (CDS) process were used to study the high susceptibility of the aluminum alloy AA 7068 to hot-tearing. A precise analysis of the measured hot-tearing curves is provided considering the casting design and different feeding mechanisms. Scanning Electron Microscopy was used to study the hot-tear surfaces. It was found that the solidification of the eutectic liquid at the film stage and the phenomenon of solid-feeding strongly affects the hot-tear formation. The early formation of an absorbed eutectic layer on the primary phase due to low rate of back-diffusion is proposed to explain the loss of ductility at the film stage. The layer can serve as an efficient substrate for the eutectic nucleation and growth, hence its formation can advance the eutectic solidification. The isolation of liquid eutectic by its partial solidification results in development of solid-feeding stresses breaking the solid eutectic and provide the required tearing initiators. By increasing the back-diffusion rate, the CDS process avoids formation of the adsorbed solid layer which postpones eutectic solidification and thereby mitigating the hot-tearing susceptibility.

Effects of Native AlN Particles on Heterogeneous Nucleation in an Al-3Fe Alloy

The naturally formed inclusions in an Al-3Fe alloy were investigated using a pressurized melt filtration technique. Naturally formed AlN particles with a rodlike morphology and a size distribution between 200 nm and 4 µm coexist with the native Al2O3 particles in the Al-3Fe alloy. These native AlN particles were investigated with scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). A well-defined orientation relationship (OR) between AlN and α-Al was found from the AlN particle embedded in α-Al. A tilt angle of 5.8 deg was observed between {111}α-Al and {0001}AlN at the α-Al/AlN interface. The atomic matching for the α-Al/AlN interface was investigated by considering interfacial segregation on the AlN particles. The heterogeneous nucleation potency of the native AlN particles was investigated. Although these native AlN particles can nucleate the α-Al, the contribution is very small. This contribution reveals the heterogeneous nucleation of α-Al on the native AlN particles and the competitive relationship between the native Al2O3 and AlN as the nucleation substrates. The surface modification of the native AlN in Al alloys containing multiple alloy elements and the corresponding effects on the heterogeneous nucleation were preliminarily investigated.

Effects of modifying the hypoeutectic AlMg5Si2Mn alloy via addition of Al10Sr and/or Al5TiB

This work demonstrates that the combined addition of Al10Sr and Al5TiB master alloys to the AlMg5Si2Mn effectively refines the grain microstructure and partially modifies the eutectic Mg2Si phase. Thorough spectroscopic characterization reveals that the grain refinement effect is due to Al3Ti particles acting as nucleation sites for α-Al grains, and the increased nucleation temperature of α-Al is due to Al10Sr addition. It is also determined that TiB2 particles can act as nucleation substrates for the primary Mg2Si phase. The prepared alloy sample with the finest microstructure (treated with both Al10Sr and Al5TiB) exhibits the greatest corrosion resistance among all tested samples.

Revealing nanoscale mineralization pathways of hydroxyapatite using in situ liquid cell transmission electron microscopy.

To treat impairments in hard tissues or overcome pathological calcification in soft tissues, a detailed understanding of mineralization pathways of calcium phosphate materials is needed. Here, we report a detailed mechanistic study of hydroxyapatite (HA) mineralization pathways in an artificial saliva solution via in situ liquid cell transmission electron microscopy (TEM). It is found that the mineralization of HA starts by forming ion-rich and ion-poor solutions in the saliva solution, followed by coexistence of the classical and nonclassical nucleation processes. For the nonclassical path, amorphous calcium phosphate (ACP) functions as the substrate for HA nucleation on the ACP surface, while the classical path features direct HA nucleation from the solution. The growth of HA crystals on the surface of ACP is accompanied by the ACP dissolution process. The discoveries reported in this work are important to understand the physiological and pathological formation of HA minerals, as well as to engineer the biomineralization process for bone healing and hard tissue repairs.

Molecular Dynamics Study of Bubble Nucleation on an Ideally Smooth Substrate.

Questions regarding bubble nucleation on an ideally smooth surface are seemingly endless, but it can not be adequately verified yet because of the scale limitation (microscopic scale). Hence, in this study, bubble nucleation on an ideally smooth substrate is explored using the molecular dynamics simulation method. An ideally smooth hydrophilic platinum substrate at 145 K is conducted to heat the simple L-J liquid argon. Results show that a visible bubble nucleus successfully forms on the ideally smooth substrate without any additional disturbance, which is common in boiling studies using the traditional numerical simulation methods. However, the nucleation position is unpredictable. At the atomic level, the thermal energy transfer from an ideally smooth substrate to liquid atoms is inhomogeneous due to atomic inhomogeneous distribution and irregular movement, which are the key influencing factors for achieving bubble nucleation. The inhomogeneity will be highlighted with the heating process. As a result, some local liquid atoms near the ideally smooth surface absorb more thermal energy to overcome their potential barrier at a specific moment, causing the emergence of a distinct nucleus there. Furthermore, nanostructure substrates are introduced to make a comparison with the smooth substrate in bubble nucleation. There is no significant difference in the inception temperature of nucleation between the ideally smooth and nanostructure substrates, but the latter has better performance in improving the bubble nucleation rate.

Atomic Ordering at the Liquid-Al/MgAl2O4 Interfaces from Ab Initio Molecular Dynamics Simulations

MgAl2O4 spinel particles exist inevitably in Al-Mg alloy melts and may act as potential substrates for heterogeneous nucleation of solid aluminum during solidification processing. In this paper, we investigated systematically the atomic ordering of liquid Al adjacent to liquid-Al/MgAl2O4{1 1 1} interfaces using an ab initio molecular dynamics simulation technique. Our simulations revealed that the interaction between the liquid metal and the spinel surface results in the formation of an ordered metal layer that terminates the substrate. This new terminating layer is positively charged, chemically bonded to the substrate, topologically rough, and structurally coupled with the metal sublayers beneath the outmost oxygen layer. The present results may shed new light on the role of spinel particles in Al-Mg alloys and on heterogeneous nucleation processes in general.

Grain refinement of Mg-Al alloys inoculated by MgAl2O4 powder

As a potent nucleating substrate for α-Mg grain, MgAl 2 O 4 powder was used to inoculate the Mg-Al melt in this study. The effects of MgAl 2 O 4 amount, holding time and Al content on the grain size and grain refining ratio of the inoculated Mg-Al alloys are systematically investigated. The results show that the minimum grain size of Mg-3Al alloy is achieved by adding 2 wt.% MgAl 2 O 4 powder and this alloy exhibits higher grain refining ratio than Mg-5Al and Mg-8Al alloys. The crystallographic misfit calculation indicates the well-matching and possible orientation relationships (ORs) between α-Mg and MgAl 2 O 4 . Among these predicted ORs, [10–10] α − Mg //[110] MgAl 2 O 4 in (0002) α − Mg //(1–13) MgAl 2 O 4 possesses the smallest misfit, i.e., 2.34% ( f r ). Both results of the experiment and crystallographic calculation demonstrate that the grain refinement of Mg-Al alloys is attributed to the MgAl 2 O 4 particles acting as the heterogeneous nucleation substrates for α-Mg grains.

Effects of CeO2 on the Si Precipitation Mechanism of SiCp/Al-Si Composite Prepared by Powder Metallurgy.

SiCp/Al-Si composites with different CeO2 contents were prepared by a powder metallurgy method. The effect of CeO2 content on the microstructure of the composites was studied. The mechanism of CeO2 on the precipitation of Si during sintering was analyzed by theoretical calculations. The results show that the appropriate amount of CeO2 can significantly refine the size of precipitated Si particles in the composite and increase the number of Si particles. With the increase of CeO2 content from 0 to 0.6 wt%, the number of Si particles precipitated in the composites increases gradually, and the average particle size of Si particles decreases gradually. When the CeO2 content is 0.6 wt%, the number of Si particles precipitated in the composites reaches the maximum, and the average particle size reaches the minimum. However, with the increase of CeO2 content from 0.6 wt% to 1.8 wt%, the number of Si particles precipitated in the composites began to decrease, and the average size of Si particles gradually increased. CeO2 can be used as heterogeneous nucleation substrate of precipitated Si, and the nucleation rate of precipitated Si on a CeO2 substrate is higher than that on an aluminum substrate. The proper addition of CeO2 can improve the nucleation efficiency of precipitated Si, thus increasing the amount and refining the size of precipitated Si.

Effect of Ti and Zr elements with equal mass ratio on microstructure and corrosion resistance of Zn‐11Al‐3Mg alloy

AbstractIn this study, the effect of Ti and Zr elements with equal mass ratio on microstructure and corrosion resistance of Zn‐11Al‐3Mg alloy was investigated. The microstructure was significantly refined and Al‐rich phase transformed from dendrite to petal‐like with the addition of Zr and Ti elements, due to the Al3(TixZr1 − x) phase as the nucleation substrate. The corrosion resistance of Zn‐11Al‐3Mg‐x(Ti,Zr) alloy was effectively improved. Moreover, the corrosion products of Zn‐Al‐Mg alloy were not changed by the addition of Ti and Zr, which are mainly composed of Zn5(OH)8Cl2·H2O and Zn6Al2(OH)16CO3·4H2O.

Effective membrane backwash with carbon dioxide under severe fouling and operation conditions

Membrane fouling is an intrinsic deficiency common for all membrane processes. Fouling mitigation is therefore required to achieve sustainable membrane performance. Our study suggests a novel backwash concept which utilizes network of ultrafiltration interconnected membrane pores as a substrate for CO2 bubbles nucleation from a saturated CO2 solution which enters membrane pores from the support side. As a result, enhanced fouling removal was achieved due to additional hydrodynamic forces caused by expanded and lifted CO2 bubbles. An investigation of CO2 nucleation kinetics using a high speed camera revealed that initial CO2 nucleation rate is strongly determined by the module height and feed water type. A saturated CO2 solution backwash effectively removed bovine serum albumin (BSA) which caused both internal and external membrane fouling. A fouling reduction was also observed in BSA/seawater matrix opposite to cake layer buildup observed experiments with Milli-Q backwash. CO2 nucleation allowed to remove hydraulically irreversible fouling which was caused by transparent exopolymer particles (TEP) at pH 4 and 8. This is a promising result as TEP is biofouling precursor which tends to adsorb to a membrane surface making conventional cleaning practices inefficient. Complete transmembrane pressure recovery was achieved with a feed water containing sodium alginate and SiO2 nanoparticles with sizes compatible with membrane pores. The observed results emphasized the importance of the specific interactions in membrane/foulant/CO2 bubble triangle for a successful membrane recovery.